1. Field of the Invention
This invention relates to devices for generating near-field light sources. More particularly, the present invention relates to methods and structures for generating multiple, independently controlled near-field light sources at subwavelength resolutions.
2. Description of the Related Art
Near-field light sources are useful for generating subwavelength, intense light sources for use in optical microscopes, optical measurement instruments, spectroscopic instruments, optical recording and optical reproduction equipment, lithography equipment, and for thermally assisted magnetic recording. In the latter application, heat is applied to a magnetic substrate via a very small, but intense light source to reduce the anisotropy of fine grain magnetic structures. These fine grain structures are capable of high recording densities, but have an anisotropy at room temperature that is too high for typical fields produced by conventional recording heads. Heating allows the media to be written with field strengths of conventional heads. However, to be useful for high density recording, the light source utilized for heating must be on the order of 20 to 30 nm in diameter. This is far beyond the optical diffraction limit for conventional light sources such as solid state lasers. Another application of interest for near-field light sources is direct writing photolithography. Since these near-field sources can produce resolutions significantly better than the diffraction limit, finer semiconductor structures can be produced.
One method that can be used to produce such a near-field light source is the ridge aperture of FIG. 1a,b (Prior Art). FIG. 1a (Prior Art) is a schematic plan view 100 of a typical ridge aperture. The device consists of an aperture 106 of length 110 and width 112 placed in an electrically conductive metal film 102. The open area of the aperture 106 is a dielectric film 104. Metal film 102 is supported on a transparent substrate (not shown). The substrate is transparent to the wavelength of incident radiation, and covers the area of aperture 106. Extending into the center portion of aperture 106 is electrically conductive ridge 108, having a width 116 and length 114. The distance between the end of ridge 108 and the side of the aperture opposite to the ridge is gap 118. Ridge 108 is a conductive material which generally is an extension of metal film 102. If the incident radiation (not shown) is polarized in the direction of arrow 122, which is perpendicular to length dimension 114 of ridge 108, little or no near-field light is created. FIG. 1b is a schematic plan view 101 of aperture 106 having the incident radiation (not shown) polarized in a direction indicated by arrow 120, which is parallel to length dimension 114 of ridge 108. In this case, localized plasmons produced by the incident radiation create a near-field light source 124, which appears close to or at the end of ridge 108. Light source 124 can be considerably brighter than the incident radiation passing through the transparent aperture 106. For example, typical dimensions for the near-field aperture of FIG. 1a are length 110=220 nm; width 112=300 nm; ridge length 114=120 nm, ridge width 116=20 to 30 nm; which produces a near-field light source about 20 to 30 nm in diameter. A ridge aperture of this type that delivers near-field light can be designed for input wavelengths range from infrared to ultra violet.
Often, it is desirable to have a plurality of near-field light sources, spaced closely together (within tens of nanometers, for example), and independently controllable of each other. Such sources can be used for alternate and simultaneous track writing in thermally assisted magnetic recording, or for high speed direct write lithography applications. While the prior art illustrates that arrays of near-field apertures, such as those in FIG. 1, can be fabricated, these arrays are usually spaced such that the near-field sources generated are at least the width or length of the aperture apart (on the order of hundreds of nanometers). Additionally, the arrays are oriented in such a manner as to have all light sources on or all off, depending on the direction of the polarization of the incident radiation. What is needed is a method and structure to produce multiple, closely spaced near-field light sources that are independently controllable.
U.S. Pat. No. 5,696,372 discloses a near-field electromagnetic probe that converts an incident energy beam into an interrogating beam which exhibits, in the near-field vicinity of the probe, a transverse dimension that is small in relation to the wavelength of the incident energy beam. The probe comprises an energy source for providing the incident energy beam with a wavelength λ. An antenna is positioned in the path of the incident energy beam and comprises at least a first conductive region and a second conductive region, both of which have output ends that are electrically separated by a gap whose lateral dimension is substantially less than λ. The electromagnetic system which produces the incident energy should preferably have its numerical aperture matched to the far-field beam pattern of the antenna. Further, the incident beam should have a direction of polarization which matches the preferred polarization of the antenna. The near-field probe system of the invention can also sense fields in the near-field gap and reradiate these to a far-field optical detector. Thus the probe can serve to both illuminate a sample in the near-field gap, and to collect optical signals from an illuminated sample in the near-field gap.
U.S. Pat. No. 6,649,894 discloses an optical near-field probe of high resolution and high efficiency. A near-field light is generated using a tapered, plane scatterer formed on a substrate surface. The intensity of the near-field light is enhanced by making the area of the scatterer smaller than that of a light spot and by selecting the material, shape, and size of the scatterer so as to generate plasmon resonance. An optical near-field generator having a high light utilization efficiency can be obtained.
U.S. Pat. No. 6,714,370 discloses a recording head for use in conjunction with a magnetic storage medium, comprising a waveguide for providing a path for transmitting radiant energy, a near-field coupling structure positioned in the waveguide and including a plurality of arms, each having a planar section and a bent section, wherein the planar sections are substantially parallel to a surface of the magnetic storage medium, and the bent sections extend toward the magnetic storage medium and are separated to form a gap adjacent to an air bearing surface, and applies a magnetic write field to sections of the magnetic recording medium heated by the radiant energy. A disc drive including the recording head and a method of recording data using the recording head are also provided.
U.S. Pat. No. 6,768,556 discloses a near-field probe including a metallic scatterer fabricated on a substrate in a contour of a circular cone, a polygonal pyramid, a planar ellipse, or a triangle and a film of a metal, a dielectric, or a semiconductor formed in a periphery of the scatterer with film thickness equal to height of the scatterer.
U.S. Pat. No. 6,785,445 discloses a near-field light probe capable of emanating a near-field light having a sufficient intensity while allowing reduction of aperture size to improve resolution. The near-field light probe can be incorporated in a near-field optical microscope, a near-field light lithography apparatus, and a near-field light storage apparatus. A near-field light probe has a configuration in which a light-blocking film is formed with an aperture having slits surrounding the major opening. Light emitted from a light source is coupled into the probe from one side of the light-blocking film, the light being polarized in a predetermined direction with respect to the slits so that a near-field light emanates from the major opening.
U.S. Pat. No. 6,795,380 discloses a pair of members opposed to each other via a gap which are commonly used as an evanescent light probe and a writing magnetic head. When the spacing and width of the gap are smaller than the wavelength λ of injected light, highly intensive evanescent light is generated from the gap position of the opposite surface. Magnetic writing is carried out by applying a recording magnetic field from the pair of members to a medium heated by the evanescent light.
U.S. Pat. No. 6,839,191 discloses an optical near-field generating element provided with: a light shielding member, which is placed on an optical path of light emitted from a light source, for defining a micro opening having a diameter equal to or shorter than a wavelength of the light; and a dielectric film placed in close contact with the micro opening. Alternatively, an optical near-field generating element is provided with a light shielding member, which is placed on an optical path of lights emitted from a light source, for defining a micro opening having a diameter equal to or shorter than a wavelength of the light, the shielding member equipped with: a main portion for defining a basic shape of the micro opening; and a protrusion portion protruding from the main portion toward the center of the micro opening.
US Patent Application Publication 2003/0015651 discloses optical apparatuses using the near-field light where high spatial resolution and high sensitivity are made compatible. Highly intense near-field light is generated in a narrow area using localized plasmons that are produced in a metal pattern in the shape that bears anisotropy and is made to irradiate a measured subject. The direction of polarization of incident light is modulated and signal light is subjected to synchronous detection, so that background light is removed and high sensitivity is achieved.
US Patent Application Publication 2003/0223316 discloses a recording head for decreasing recording noise accompanying malformation of a recorded mark and the formation of a recorded mark capable of increasing reproduction resolution at the time of magnetic reproduction. The head has a light source and a scatterer for recording information on a recording medium by generating near-field light by application of light from the light source and forming a magnetic domain array on the recording medium, a perimeter of the scatterer defines a plurality of vertices and a distance between a first vertex and a last vertex is shorter than the width of the recording track on the recording medium. The recording head improves recording density and can be used to manufacture a highly reliable information recording and reproducing apparatus having a reduced cost per capacity.
US Patent Application Publication 2004/0062152 discloses a device for writing data to a recording medium and a method for fabricating the device. According to one embodiment, the device includes an electrical conductor having a cross-track portion, wherein the cross-track portion includes first and second opposing surfaces, and wherein the cross-track portion defines an aperture extending from the first surface to the second surface. The device also includes a dielectric portion disposed in the aperture such that the dielectric portion defines a ridge waveguide having a lowest-order mode cut-off frequency that is less than the frequency of incident optical energy used to heat the recording medium.
International Publication WO 01/17079 discloses a near-field optical apparatus comprising a conductive sheet or plane having an aperture therein, with the conductive plane including at least one protrusion which extends into the aperture. The location, structure and configuration of the protrusion or protrusions can be controlled to provide desired near-field localization of optical power output associated with the aperture. Preferably, the location, structure and configuration of the protrusion are tailored to maximize near-field localization at generally the center of the aperture. The aperture preferably has a perimeter dimension which is substantially resonant with the output wavelength of the light source, or is otherwise able to support a standing wave of significant amplitude. The apparatus may be embodied in a vertical cavity surface emitting layer or VCSEL having enhanced near-field brightness by providing a conductive layer on the laser emission facet, with a protrusion of the conductive layer extending into an aperture in the emission facet. The aperture in the emission facet preferably has dimensions smaller than the guide mode of the laser, and the aperture preferably defines different regions of reflectivity under the emission facet. The depth of the aperture can be etched to provide a particular target loss, and results in higher optical power extraction from the emission facet.
Sendur et al., in an article entitled “Ridge waveguide as a near-field aperture for high density data storage”, Journal of Applied Physics, Volume 96, No. 5, September 2004, discloses the performance of the ridge waveguide as a near-field aperture in data storage systems. Finite element method (FEM) and finite-difference time-domain (FDTD) based software are used in the numerical simulations. To verify the accuracy at optical frequencies, the FEM and FDTD are first compared to analytical results. The accuracy of these techniques for modeling ridge waveguides at optical frequencies is also evaluated by comparing the results with each other for a plane wave illumination. The FEM, which is capable of modeling focused beams, is then used to simulate various geometries involving ridge waveguides. Near-field radiation from ridge waveguide transducer is expressed in terms of power density quantities. Previous studies in the literature consider the performance of the transducer in free space, rather than in the presence of a recording magnetic medium. The effect of the recording magnetic medium on the transmission efficiency and spot size is discussed using numerical simulations. The effect of various geometric parameters on the optical spot size and transmission efficiency is investigated and discussed. Based on the numerical simulations, a promising transducer design is suggested to obtain intense optical spots well below the diffraction limit. Numerical simulations suggest that a full width at half maximum spot diameter of 31 nm in the recording magnetic medium can be obtained. The maximum value of the absorbed optical power density in the recording medium is about 1.67×10−4 mW/nm3 for a 100 mW input power. In-track and cross-track profiles for this design are compared with Gaussian distributions.